In cryogenic liquid tanks, such as propellant tanks for aerospace vehicles, it is desirable to know the temperature and material state in various levels of the tank. As the density of the liquid (as well as headspace vapor) varies with temperature, this information is important in determining such values as the mass of the liquid in the tank. The conditions to which the tanks, and the corresponding probes, are subjected include cryogenic temperatures and elevated pressures and may include fluctuations in temperature, pressure and the like. It is therefore highly desirable to ensure that the juncture at which the probe enters the tank be effectively sealed and that the seal be maintained under such extreme conditions.